Abstract The permeability reduction due to the coal matrix swelling induced by the adsorption of CO2 is the major concern during enhanced coalbed methane recovery (ECBM) as well as CO2 sequestration in coal reservoir. Many problems arise in the context of this process regarding the coal permeability variations, and very few studies have been made with consideration of long-term CO2 saturations on gas flow behaviour alterations. The main objective of this study is therefore to examine the influence of both subcritical and supercritical CO2 flooding durations on N2 and CO2 permeability variation of naturally fractured low volatile bituminous coal under various tri-axial conditions. Five CO2 flooding durations, up to 65 h, both subcritical (6 MPa) and supercritical (14, 20 MPa), were introduced to the sample under three confinements, respectively. Results show that for 11 MPa and 17 MPa confining environment, most permeability reduction occurs during the first CO2 flooding process, and supercritical CO2 flooding causes significant higher N2 permeability reduction (from 14.38% to 50.18%) than subcritical CO2 (from 4.11% to 11.25%). The permeability reduction rate decreases with time but this reduction continues even after a total of 153 h flooding. Longer permeability reduction process is observed upon supercritical CO2 adsorption due to the much higher viscosity of supercritical CO2. Under deep underground condition (>1 km), CO2 exhibits a N2-like flow behaviour because of the dominant poroelastic effect. The influence of CO2 flooding on permeability alterations has been significantly compromised with much smaller permeability reduction after 20 MPa CO2 flooding compared with 14 MPa CO2 flooding (7.33% and 38.75% for 7 MPa CO2 injection pressure, respectively), along with no apparent permeability reduction for the further CO2 flooding scenarios.

中文翻译：

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CO2饱和时间对煤气流量的影响：压裂烟煤

摘要 CO2 吸附引起的煤基质膨胀导致渗透率降低是提高煤层气采收率（ECBM）以及煤储层 CO2 封存过程中的主要问题。在这个过程中出现了许多关于煤渗透率变化的问题，很少有研究考虑到长期 CO2 饱和度对气流行为改变的影响。因此，本研究的主要目的是检验亚临界和超临界 CO2 驱替持续时间对不同三轴条件下天然裂缝性低挥发性烟煤 N2 和 CO2 渗透率变化的影响。五个 CO2 驱油持续时间，最长 65 小时，亚临界 (6 MPa) 和超临界 (14, 20 MPa)，分别在三个限制条件下引入样品。结果表明，对于 11 MPa 和 17 MPa 围压环境，大部分渗透率降低发生在第一次 CO2 驱过程中，超临界 CO2 驱使 N2 渗透率降低（从 14.38% 到 50.18%）显着高于亚临界 CO2（从 4.11% 到 11.25） %）。渗透率降低率随着时间的推移而降低，但即使在总共驱油 153 小时后，这种降低仍在继续。由于超临界 CO2 的粘度更高，因此在超临界 CO2 吸附时观察到更长的渗透率降低过程。在深层地下条件下（> 1 km），由于主要的多孔弹性效应，CO2 表现出类似 N2 的流动行为。